Fluid flow meters operate based upon the principle that the propagation velocity of a wave in a fluid, as determined by measuring the phase shift of the signal, is indicative of the velocity of the fluid. U.S. Pat. No. 3,894,431 to Muston, et al discloses determining fluid flow rates “by transmitting ultrasonic pulses in both directions along a path through the fluid aligned with the direction in which velocity component is to be measured. Transmission of, and measurement upon, pulses in the two directions are controlled by a master clock pulse generator. The frequency of a first variable frequency oscillator is adjusted to fit N pulses exactly into the timer period for flight of an ultrasonic pulse along the path in one direction. The frequency of a second variable frequency oscillator is adjusted to fit N pulses exactly into the time period for flight of an ultrasonic pulse along the path in the opposite direction. The difference frequency is proportional to velocity component. This system may be combined with a limited sing-around system to improve resolution, at the expense of the time response.”
U.S. Pat. No. 4,885,942 to Magori discloses using “the phase difference method wherein two ultrasound transducers W1 and W2 are mounted offset but aligned with each other in a tube through which the velocity of flow is to be measured wherein both of the ultrasound transducers are excited in a pulse manner by an oscillator OS2 and wherein receiving amplifiers V1 and V2 are, respectively, associated with the ultrasound transducers W1 and W2. Evaluation devices are connected after amplifier V1 and V2 such that the phase relationship of the signals at the outputs of the receiving amplifiers V1 and V2 is determined during the reception of ultrasound signals. The phase relationship between the signals at the ultrasound transducers is also determined during transmission of ultrasound signals and this phase difference is used as a reference during reception of ultrasound signals.”
U.S. Patent Publication No. 2006/0106308 describes use of thermal measurements to detect and/or measure the reestablishment of blood flow during a clot dissolution treatment. A catheter 10 is positioned through a clot 90 at a treatment site 88 in a patient's vasculature 86. The catheter 10 includes at least an upstream thermal source 120 and a downstream thermal detector 122. When the thermal source 120 supplies thermal energy into the surrounding environment, a “thermal pulse” 124 is created therein. As the thermal pulse 124 propagates downstream, the characteristics of the thermal pulse 124 will change, which can be measured and analyzed using the thermal detector, thereby providing information about blood flow at the treatment site.
U.S. Patent Publication No. 2003/0056585 describes a thermal flow meter with a flow rate detecting unit containing a heating element, a flow rate detecting temperature sensing element and a flow rate detecting electroconductive heat transfer member extending into a fluid flow passage, which are disposed so as to enable heat transfer therebetween, the flow rate detecting temperature sensing element varying in electrical characteristic value in accordance with flow of a fluid in the fluid flow passage through heat exchange with the fluid in the fluid flow passage which is carried out through the flow rate detecting electroconductive heat transfer member; and a fluid temperature detecting unit containing a fluid temperature detecting temperature sensing element and a fluid temperature detecting electroconductive heat transfer member extending into the fluid flow passage, which are disposed so as to enable heat transfer therebetween, the fluid temperature detecting temperature sensing element varying in electrical characteristic value in accordance with the temperature of the fluid through heat exchange with the fluid in the fluid flow passage, wherein a flow rate of the fluid is detected on the basis of the electrical characteristic value of the flow rate detecting temperature sensing element and the electrical characteristic value of the fluid temperature detecting temperature sensing element, and fluid discrimination is effected by determining a conductivity between the flow rate detecting electroconductive heat transfer member and the fluid temperature detecting electroconductive heat transfer member. All of the aforementioned patents and published applications are herein incorporated by reference.
These aforementioned prior art flow meters often suffer from excessive noise and may have limited efficacy where space around the flowing fluid is limited (such as in conduits of small diameters). These conditions are typically true for medical applications and are particularly so in extracorporeal blood processing systems such as hemodialysis, hemofiltration and hemodiafiltration systems.
Accordingly, there is need in the art for a thermal flow meter that has improved accuracy and efficiacy. There is also a need in the art for a thermal flow meter that has decreased sensitivity to noise and signal dispersion. Finally, there is a need in the art for a thermal flow meter that can be readily implemented in a manifold, does not require expensive, non-disposable materials, and can generate a signal that can be readily, and easily, filtered.